Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Calculations for basic and diluted earnings per common share
Net income	$ 53,816	$ 49,438	$ 145,441	$ 145,510
Preferred stock dividends	(2,188)	(2,184)	(6,555)	(4,957)
Net income available to common equity	51,628	47,254	138,886	140,553
Common shareholder dividends	(16,431)	(14,927)	(49,077)	(45,149)
Unvested share-based payment awards	(177)	(164)	(565)	(450)
Undistributed earnings	35,020	32,163	89,244	94,954
Undistributed earnings allocated to common shareholders	34,645	31,813	88,294	93,961
Undistributed earnings allocated to unvested share-based payment awards	375	350	950	993
Undistributed earnings	35,020	32,163	89,244	94,954
Basic
Distributed earnings to common shareholders	16,431	14,927	49,077	45,149
Undistributed earnings allocated to common shareholders	34,645	31,813	88,294	93,961
Total common shareholders earnings, basic	51,076	46,740	137,371	139,110
Diluted
Distributed earnings to common shareholders	16,431	14,927	49,077	45,149
Undistributed earnings allocated to common shareholders	34,645	31,813	88,294	93,961
Total common shareholders earnings, diluted	$ 51,076	$ 46,740	$ 137,371	$ 139,110
Weighted average common shares outstanding (in shares)	148,708	148,614	148,607	149,524
Effect of dilutive common stock awards (in shares)	1,265	1,185	1,038	1,180
Diluted weighted average common shares outstanding (in shares)	149,973	149,799	149,645	150,704
Basic earnings (loss) per common share (in usd per share)	$ 0.34	$ 0.31	$ 0.92	$ 0.93
Diluted earnings (loss) per common share (in usd per share)	$ 0.34	$ 0.31	$ 0.92	$ 0.92
Earnings Per Share (Textuals) [Abstract]
Number of antidilutive stock options excluded from the computation of diluted earnings per share (in shares)	1,000	1,000	1,000	1,000
Number of antidilutive warrants excluded from the computation of diluted earnings per share (in shares)	4,000	4,000	4,000	4,000